1. Field of the Invention
This invention relates to a post for guiding tape in a magnetic tape drive. More particularly, the invention is a guide post including a tape engaging surface having a longitudinal contour with a compound radius of curvature.
2. Description of the Related Art
Web and tape guiding apparatus are well known. Perhaps the most common use of such an apparatus is for guiding tapes in magnetic tape drives over a magnetic read/write head. Guiding of the tape over the head is critical to the writing of data to and the reading of data from the magnetic tape. A typical magnetic tape drive includes several elements in the tape path to ensure proper alignment and operation of the tape and head. For example, the tape path of the IBM 3480/3490 tape drives includes a supply reel inside a tape cartridge, a vibration decoupler, a cleaner blade, an arcuate supply side air bearing device, a magnetic read/write head, an arcuate storage side air bearing device, a tension transducer, a storage reel, and tape edge guides along the sides of the bearing surfaces. The tape edge guides are located along the arcuate bearing surfaces because the tape in such proximity can support a larger guiding force without collapsing than can a freely suspended tape. The tape edge guides physically align the lateral position (i.e. in the direction of the tape width) of the tape relative to the read/write head. Lateral positioning maintains proper alignment between the read/write elements of the head and the data tracks on the magnetic tape. The tape guides also support and guide the tape in its easy direction of bending (i.e. in the direction of tape travel).
Three basic types of guides are known for easy direction tape guiding. The first such guide is a fixed cylindrical post. The use of this type of guide is based on the theory that a hydrodynamic film of air forms between the tape and the post when the tape is in motion, thereby reducing friction and wear. However, practically sized posts fail to produce an air film of sufficient thickness to reduce friction and wear to a significant extent. Also, when the tape is stopped and rests on the post under tension it sticks to the post. Such "stiction" makes the initiation of tape movement difficult and may result in damage to the tape.
Another type of easy direction tape guide is a roller. As compared to posts, rollers reduce friction with a tape in motion and stiction with a tape at rest. However, rollers introduce another source of vibration into the tape path (which may disrupt the head-tape interface). Also, rollers store energy that must be positively controlled to maintain adequate closed loop tension control. Finally, rollers steer tape differently than do posts, and often require grooves or other geometries to prevent the tape from flying over them.
The third type of easy direction tape guide is an externally pressurized air bearing. Although an air bearing can dramatically reduce tape friction and stiction, it is more complex and expensive than a post or a roller. An air bearing is complex because it requires the space and parts for air to be provided under controlled pressure. An air bearing is expensive because of such complexity, and because a source of compressed air must be provided. Finally, more than one type of easy direction guide can be used in a tape path to form a variety of tape path configurations.
Examples of fixed cylindrical posts and/or rollers are disclosed in U.S. Pat. Nos. 4,341,335, 3,360,174 3,276,651, 3,327,964, 4,144,991, 4,633,347, and 3,991,956. To the extent that these references disclose the guiding of flexible webs, the posts and/or rollers are used to drive web movement, control web tension, or provide lateral web guiding. In addition, U.S. Pat. Nos. 2,095,733, 4,335,857, 3,393,849, 3,132,788, 4,913,328, and 4,084,683 disclose rollers having a lateral contour designed to guide a web laterally. Air bearings are disclosed in U.S. Pat. No. 4,071,177, Garcia et al, Compliant Guide Assembly with High Wear Resistance Contact Pads, IBM Technical Disclosure Bulletin, Vol 29, No. Oct. 5, 1986, pp. 2126-27, and in the IBM 3480/3490 tape drives. Also, U.S. Pat. No. 4,276,575 discloses the use of a plastic lubricant on a tape guide to reduce friction. However, for the aforementioned reasons, these references fail to achieve a simple, low cost, low friction/stiction tape guide which accommodates changes in tape direction (i.e. easy direction bending).